A fuel cell is highly expected as energy for the next generation which is clean and can exhibit a high power generation efficiency, and particularly in recent years, a polymer electrolyte fuel cell attracts attention because of the advantages that a high output power is obtained and that an operating temperature range is relatively low.
The above polymer electrolyte fuel cell is usually constituted from a stack in which a unit cell is laminated, comprising a polymer electrolyte membrane comprising an ion-exchange membrane, two electrodes provided at both sides of the polymer electrolyte and a separator provided therein with gas-feeding grooves for feeding a fuel gas such as hydrogen or an oxidizing gas such as oxygen to the respective electrodes and two power collectors provided at the outsides of the stack.
In the above fuel cell, a high output power can be generated, though an operating temperature is as low as 80 to 100° C., since a polymer electrolyte membrane having a high performance is used at an electrolyte part.
High gas impermeability is required to the separator of the above polymer electrolyte fuel cell in order to feed a fuel gas and an oxidizing gas to the electrodes in a completely separated state. Also, an internal resistance of the cell has to be reduced in order to enhance the power generation efficiency, and therefore the separator has to have the high electrical conductivity. Further, excellent corrosion resistance, chemical resistance, mechanical strength and hydrophilicity have to be endowed in order to secure long term durability and high thermal conductivity for efficiently releasing heat generated as cell reaction proceeds to uniformize temperature distribution in the cell.
Among the above characteristics required, a performance of making it possible to quickly discharge water produced in power generation, that is, the presence of hydrophilicity in the separator is one of the most important characteristics required.
Many arts have so far been known as art for enhancing hydrophilicity of a separator for a fuel cell, and known are, for example, (1) a separator for a polymer electrolyte fuel cell which is prepared by subjecting conductive carbon having a hydrophilic functional group on at least a part of a surface and a binder to molding by pressing or hot molding by pressing and in which irregularities of average 50 μm or more and 1 mm or less are provided on a surface of a gas passage by sand blast processing (refer to, for example, patent document 1), (2) hydrophilicity treatment of a separator for a fuel cell characterized by subjecting the separator for a fuel cell to atmospheric discharge plasma treatment (refer to, for example, patent document 2) and (3) a separator for a fuel cell which is prepared by molding a composition comprising a thermosetting resin, artificial graphite having an average particle diameter of 20 to 70 μm and an internal mold releasing agent and which is provided with an average surface roughness Ra of 1.0 to 5.0 μm produced by a surface treatment method such as a shot blast method and the like (refer to, for example, patent document 3).
However, the separators for a fuel cell in which uneven parts are formed by the sand blast processing, the atmospheric discharge plasma treatment and the shot blast method each described in the patent documents 1 to 3 described above involve the problem that since the uneven parts are formed on the surface of the gas passage, parts other than the uneven parts have to be masked so that the surface treating step is complicated and the problem that the uneven parts are not accurately formed so that the yield is lowered. Further, involved therein is the problem that the hydrophilic performance is still reduced with the passage of time in the above surface treatments.
Further, in the separator for a fuel cell described in the patent document 1 described above, the carbon particles are subjected to treatment for providing with a hydrophilic functional group and then subjected together with a binder to molding by pressing or hot molding by pressing, and after molding, treatment in which the uneven parts are formed on the surface of the gas passage is carried out, so that brought about are the problems that the production efficiency is inferior and that dispersion in the performance is caused.
In the separator for a fuel cell described in the patent document 2 described above in which the plasma treatment is carried out, various treating gases are essentially required, and further there is the restriction that the treatment has to be carried out in atmosphere in which the gas is diluted by an inert gas from a safety point of view. Further, the excited gas is blown and brought into contact with the separator for a fuel cell to carry out treatment, and therefore the problem that it is difficult to carry out partial processing to form complicated and fine groove patterns is involved therein. Also, there is the problem that the wetting property is reduced with the passage of long time in a dry state. Further, the separator for a fuel cell described in the patent document 3 described above is prepared by molding a composition comprising a thermosetting resin, an artificial graphite having an average particle diameter of 20 to 70 μm and an internal mold releasing agent, and involved therein are the problem that the internal mold releasing agent added bleeds out with the passage of time and the problem that the wetting property is not satisfactory due to the composition blended with the resin and the mold releasing agent and that the wetting property is reduced with the passage of time.
On the other hand, known as art in which laser treatment is used for producing a fuel cell are, for example, a production process for a fuel cell in which an MEA is constituted by a polymer electrolyte membrane and a fuel electrode and an air electrode each provided at both sides of the electrolyte membrane and in which both sides of the MEA are interposed by separators to laminate a single cell stack, wherein the separator is molded integrally a mixture of graphite particles and a resin to provide with a gas passage on the surface, and then a skin layer formed on the separator is removed by carbonizing the skin layer having a large resin content by laser irradiation (refer to, for example, patent document 4). Further, known is a production process of an electrolyte membrane for a fuel cell comprising (a) a step for forming a substrate from hydrogen-permeable metal and (b) a step for forming a ceramic layer having proton conductivity on the surface of the substrate described above, wherein in the above step (b), crystallization energy required for crystallizing an amorphous material which forms the ceramic layer described above is supplied locally on the amorphous material by laser irradiation (refer to, for example, patent document 5).
However, in the art described in the patent document 4 described above, the groove part is masked, and the surface of the separator having a large resin content other than the groove part is carbonized to reduce the contact electric resistance. It is not perceived at all to subject the groove part to hydrophilization treatment by laser irradiation, and the above art is completely different from the present invention in an object and a technical concept (constitution and action effect). In addition thereto, a dose of the laser irradiation for carrying out the above carbonization is too large to achieve the hydrophilization treatment by the dose concerned.
Further, the art described in the patent document 5 described above is a production process of an electrolyte membrane for a fuel cell, and it is not perceived at all to carry out hydrophilization by laser irradiation, so that the above art is completely different from the present invention in an object and a technical concept (constitution and action effect).
Patent document 1: Japanese Patent Application Laid-Open No. 283873/2001 (claims, examples and others)
Patent document 2: Japanese Patent Application Laid-Open No. 25570/2002 (claims, examples and others)
Patent document 3: Japanese Patent Application Laid-Open No. 197222/2005 (claims, examples and others)
Patent document 4: Japanese Patent Application Laid-Open No. 335121/2004 (claims, examples and others)
Patent document 5: Japanese Patent Application Laid-Open No. 5088/2005 (claims, examples and others)